The former method and apparatus allowed the production of panels of phenolic foam displaying surface skins. That is the panels possessed smooth surfaces on the four main sides; and, in addition, on their upper principal face, they possessed a layer of cellular material of increased density. These skins, and particularly a skin of uniform thickness on the upper fabrication side, are advantageous in four respects. They increase by a considerable effect the resistance to abrasion; they increase the force required to rupture by bending substantially and also the maximum force required to puncture; they, finally, decrease the permeability to water vapor. The skin located on the upper side of the panels has an irregular thickness and displays, after fabrication, wavy surfaces and zones of partial tearing.
Thus the upper side of the panels displays a generally unaesthetic aspect, and its irregularity prevents obtaining any possible improvement of the resistance to abrasion, of the force required to rupture by bending, of the maximum force required to puncture exerted perpendicular to the panel, and of the permeability to water vapor. In addition, the lateral sides of the fabricated panels with the help of the apparatus described in U.S. Pat. No. 3,761,209 are irregular and in order to obtain flat and square surfaces with relation to the main sides, the borders must be cut with a saw. There results on the two lateral surfaces a complete absence of skins with the subsequent inconveniences. And finally, the apparatus described in this former patent does not allow rapidly varying the thickness of fabrication, this variation necessitating in fact the unmounting and the remounting of side pieces fixed on the edges of the lower conveyor belt with the two belts fixing the thickness of the product obtained.